Octane Number and Cetane Number.

Automotive Fuels

As we all know that the major fuels that are used in the Automobiles are Petrol in SI engines and Diesel in CI engines, apart from these there is CNG (compressed natural gas) that is also used as a fuel and the Electric Vehicles, but here we will discuss only about the Petrol/gasoline and the Diesel fuel.
The Automotive fuel which is used for the purpose of the combustion inside the engine have ratings  or we can say the quality standards on the basis of which we can easily identify the quality of fuel which will be discussed below in details:

Rating of Engine Fuel:  

In case of S.I (spark ignition) or petrol/gasoline engines, the quality or rating of fuel i.e petrol is  based on the Octane Number, whereas in case of CI (Compressed Ignition) or Diesel engines the quality of fuel (diesel) is determined by the Cetane number.

What is Octane Number?

It is an expression which indicates the ability of the fuel to resist detonation in SI engines.
It is the percentage volume of Isooctane in the mixture of isooctane and n-heptane,
  • Isooctane offers the great resistance to detonation therefore it is assigned a rating of hundred octane number, whereas n-heptane detonates very rapidly therefore it is assigned a rating of zero octane number.
  • Higher the octane number, fuel increases the efficiency of the engine because higher compression number can be used with higher octane number fuel, as higher the compression ratio higher is the Thermal Efficiency. 
"Addition of Tetra Ethyl Lead (TEL) in the gasoline helps in reducing the detonation or knocking tendency of the fuel."
Anti-knock Agents: 
Substance which decreases the rate of pre-flame reaction by delaying the auto ignition of end part of the charge in contact with the cylinder walls of the engine until flame generated by the spark plug which is the end mixture.For example: TEL, Ethyl alcohol, benzene, Fluorine etc.
Addition of TEL increases the octane reading  by 6 to 10 but it is not acceptable these days as it leads to emissions of Lead into atmosphere which is hazardous.

What is Cetane Number?

It is the Percentage volume of Cetane in a mixture of Cetane and Alpha-methyl-naphthalene that produces the same delay delay period as fuel is being tested under same conditions on the same engine.
Higher the cetane number of the fuel lesser will be the tendency for the diesel engine to Knock. Cetane has a good ignition so it is assigned a cetane reading where alpha-methyl-naphthalene has poor ignition quality so it is assigned as zero cetane reading.

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